For directional drilling of oil and gas wells, a drill string generally has, at its distal end, a drill bit which is energized by a hydraulically-powered “mud” motor. Such drill bit and hydraulically-powered motor is generally contained in a so-called “bent sub”. A universal bottom hole orienting sub assembly (“UBHO sub”), containing a directional measurement tool, is generally threadably coupled to the bent sub, uphole therefrom. The directional measurement tool within the UBHO sub contains electronic instrumentation for determining drill bit location and orientation, and together such form part of the drill string bottom hole assembly (“BHA”).
The UBHO sub is adapted at its downhole end to be threadably connected to the bent sub, and at its uphole end to be threadably connected to various other bottom hole assembly components, such as mud pulser device/assembly for transmitting downhole drilling orientation information from the directional measurement tool to surface, gamma modules for determining geology of formations being drilled through, and battery-containing elements, all of which may be connected in series to each other, and to the UBHO sub at its uphole end.
The UBHO orienting sub has a hollow cylindrical outer member (‘mule shoe’), typically having an arcuate camming surface on an interior surface thereof for landing against a mating arcuate camming surface on a directional measurement tool when such directional measurement tool is co-axially inserted and caused to nest within the outer mule shoe member. Specifically, the mule shoe member typically has an perpendicular aperture through the bore of the mule shoe, for receiving a key member which extends therethrough and into an axially extending channel in the directional measurement tool when the arcuate camming surfaces on the respective mule shoe member and inner directional measurement tool cause each of the aforesaid components to be rotated in a desired orientation one to another. The key member when inserted and pinned in such inserted position fixedly keeps such orientation of one to the other. A further set screw extending from the mule shoe member to the inner directional measurement tool in the prior art is then typically used to “lock” the directional measurement tool to the mule shoe and prevent the directional measurement tool from being removed from within the mule shoe member, and thereby allow the UBHO sub (containing the mule shoe and directional measurement tool locked therewithin) and the bent sub and drill bit attached thereto, to be lowered downhole so as to permit directional drilling.
Disadvantageously, if the drill bit should become stuck for some reason in the wellbore during directional drilling, which sometimes occurs at long wellbores of narrow diameter with extensive directional changes, and it then becomes impossible to withdraw the drill string intact, and typically the drill string will become pulled apart at a location above the UBHO and above the gamma module, mud pulser, and batteries when the drill string is attempted to be raised to surface. Such results in not only loss of the bent sub and mud motor (which in any event are “stuck”), but further results in loss of the UBHO sub containing the directional measurement tool, as well as various associated equipment uphole thereof up to the point at which the drill string parted, which typically includes such components as the gamma module, batteries, and various connecting members.
Loss of such UBHO sub and uphole equipment can be quite expensive. For example, in year 2011 (US) dollars, a directional module may cost in the range of $60,000 (US), a mud pulser $36,000, batteries in the range of $8-10,000 each, and a gamma module in the range of $25,000, with various interconnects costing in the range of $4000, and thus h the entire assembly costing in the range of approximately $125,000. Accordingly, loss of such UBHO sub and related assemblies/components downhole is quite expensive.